1. Field of the Invention
The present invention relates to a pump light source device for an optical Raman amplifier for causing signal lightwave to undergo optical Raman amplification in an optical fiber by utilizing stimulated Raman scattering and to an optical Raman amplification system using the same.
2. Description of the Related Art
As shown in FIG. 11, generally speaking, in an optical Raman amplification system, when guiding an optical signal 2 output from a light transmitting station 1 to a light receiving station 4 through a light transmission path 3 consisting of an optical fiber or the like, an optical Raman amplifier 5 is arranged at some midpoint in the light transmission path 3 or at an end thereof to cause the optical signal to undergo optical Raman amplification in order that the optical signal 2 attenuated in the light transmission path 3 may attain the requisite optical signal level for reception by the light receiving station 4.
In the light transmitting station 1, electrical information to be transmitted is converted to an optical signal before it is output to the light transmission path 3. The optical signal is prepared by directly applying an electric signal of the information to be transmitted to a semiconductor laser diode or the like constituting the signal light source or by providing behind the signal light source an external modulator by means of which the light oscillated by the signal light source is modulated by the electric signal.
In the light receiving station 4, the signal lightwave 2 propagated through the light transmission path 3 is converted to an electric signal by optical-to-electrical-converter, such as a photo diode, and the information transmitted from the light transmitting station 1 is demodulated, whereby the information is read.
As shown in FIG. 10, in the optical Raman amplifier 5, a pump light from a pump light source device 7 is transmitted to an end portion of a Raman Gain medium 31 constituting a part of the light transmission path 3 by way of an optical coupler 6 to input the pump light to the Raman Gain medium 31, in which stimulated Raman scattering is generated to thereby effect Raman amplification on the optical signal 2. The wavelength of the pump light is selected so as to be shorter than the wavelength of the optical signal 2 emitted from the light transmitting station 1 by approximately 20 to 200 nm.
The Raman gain obtained in the Raman Gain medium 31 is greatly dependent on the mutual relationship between the polarization state of the pump light and the polarization state of the signal lightwave 2. For example, assuming that the polarization of the pump light is linear polarization (Normally, a laser beam emitted from a semiconductor laser diode (LD) is in the linear polarization state), the Raman gain increases when the signal lightwave 2 is a linearly polarized light parallel to the polarization of the pump light, and it decreases when the signal lightwave 2 is a linearly polarized light perpendicular to the polarization of the pump light. This phenomenon, which is called PDG (polarization dependent gain), leads to deterioration in the bit error rate of the optical Raman amplifier.
As described above, if the polarization state of the pump light or the signal lightwave 2 fluctuates in the Raman amplification medium 31 as time passes, the gain also fluctuates the same. This fluctuation in gain is not desirable for the transmission of information.
In view of this, not less than two LDs per wavelength and a polarization beam combiner have conventionally been used in the pump light source device 7, and polarization combination has been conducted such that the states of polarization thereof are orthogonal to each other to overcome the polarization dependence of the Raman gain.
Conventionally, in order to eliminate the polarization dependence of Raman Gain, oscillating beams from light sources, e.g., semiconductor laser diodes, of the same oscillation wavelength are combined such that an orthogonal polarization state is attained and the lightwaves thus combined are input to the Raman Gain medium, or light with lower degree of polarization is input to the Raman Gain medium to thereby overcome the polarization dependence of the gain.
However, in the case of the method in which lightwaves from two light sources of the same wavelength are combined by using a polarization beam combiner, if one light source is out of order, only the other light source operates, so that there appears a fluctuation due to time in the Raman gain because of a fluctuation due to time in the polarization state of the pump light or the signal lightwave, resulting in generation of a fluctuation in amplification gain.
In the method of outputting pump light which have lower degree of polarization to gain medium, the following problems are involved when a plurality of pump lights having different wavelengths are combined.
1. When the polarization beam combiner is used for coupling pump light whose degree of polarization is low, an efficiency of coupling pump light is less than higher degree of polarization because polarization beam combiner selects the state of polarization of coupling light.
2. When using a PLC or an AWG, there is a limitation regarding the wavelength interval of the light sources, and the spectral line width of the light source is also limited to enhance the coupling efficiency.